The oscillation wavelength of a semiconductor laser varies depending on the ambient temperature and device temperature. For example, as described in K. Sakai, “1.5 μm range InGaAsP/InP distributed feedback lasers”, IEEE J. Quantum Electron, vol. QW-18, pp. 1272-1278, August 1982, the temperature dependence of the oscillation wavelength of a distributed feedback (DFB) laser is about 0.1 nm/K. This is because the refractive index (n) of a semiconductor has temperature dependence, and hence the Bragg wavelength (λB) of a diffraction grating varies according to the following expression.mλB=2nΛ  (1)where m is the order of diffraction and A is the period of the diffraction grating.
For example, when using a semiconductor laser as a light source for optical fiber communication, particularly wavelength division multiplexing communication (WDM) that transmits optical signals with different wavelengths by multiplexing them into a single fiber, the accuracy of the wavelengths of the signal light is important. Accordingly, it is essential to stabilize the oscillation wavelength of the semiconductor laser constituting the light-emitting source. To achieve this, the oscillation wavelength of the semiconductor laser is stabilized by the temperature control of the semiconductor laser using a Peltier device, for example.
Methods of stabilizing the temperature dependence of the oscillation wavelength without using the temperature control by the Peltier device or the like are broadly divided into two methods. An example of the first method is disclosed in H. Asahi et al., Jpn. J. Appl. phys., vol. 35, pp. L875-, 1996. It employs a semiconductor material having a refractive index with smaller temperature dependence than a conventional counterpart, thereby reducing the temperature dependence with a semiconductor-only configuration. A second method is one that uses a composite configuration of semiconductor and materials other than the semiconductor in order to reduce the temperature dependence. For example, the following configurations are known. One that has a semiconductor laser combined with an external waveguide composed of materials other than the semiconductor is disclosed in “Hybrid integrated external Cavity laser without temperature dependent mode hopping”, by T. Tanaka et al., Electron. Lett., vol. 35, No. 2, pp. 149-150, 1999. Another configuration that has semiconductor and non-semiconductor materials with the refractive index temperature dependence opposite to that of the semiconductor, connected alternately in cascade, is disclosed in Japanese patent application laid-open No. 2002-14247.
However, as for the method of carrying out the temperature control of the semiconductor laser with the Peltier device, it has a problem of complicating the device structure and control, and increasing the power consumption.
As for the method of reducing the temperature dependence by the semiconductor-only configuration using the semiconductor material with the refractive index of smaller temperature dependence, no reports have been made about a new material that is put to practical use, and because of the crystal growth and device formation, it is very difficult to develop such a new semiconductor.
Furthermore, as for the method of combining the semiconductors with the non-semiconductor materials, it is preferable to be able to combine them as simple as possible such as eliminating the need for optical axis adjustment. However, even if a simple fabrication method exists such as spin coating an organic material on the semiconductor substrate, in case for example of constructing distributed reflectors by alternately cascading the semiconductor and the organic materials to fabricate a first-order diffraction grating with good characteristics, it requires to place the semiconductor and organic materials alternately at about ¼ wavelength intervals, which presents a great degree of problem in the difficulty and reliability of the process.
On the other hand, by connecting a semiconductor optical waveguide with an optical waveguide composed of materials having different characteristics from the semiconductor, an optical waveguide with new characteristics is obtained which cannot be achieved by semiconductor-only. For example, while the refractive index of a semiconductor has a positive temperature dependence that increases with the temperature, a method is known which connects a semiconductor optical waveguide in cascade with an optical waveguide composed of materials whose refractive indices are negative in temperature dependence that decreases with the temperature.
As such, it is possible to implement an optical waveguide whose optical length, which is given by the product of the refractive index and the waveguide length, is independent of the temperature as a whole. For example, as disclosed in K. Tada et al., “Temperature compensated coupled cavity diode lasers”, Optical and Quantum Electronics, vol. 16, pp. 463-469, 1984, a temperature-independent laser whose oscillation wavelength is independent of the temperature can be realized by constructing its cavity from materials with the negative refractive index temperature dependence external to the semiconductor laser.
More specifically, the optical length nDLD of the laser cavity increases with the temperature because of an increase in the effective refractive index nD of the semiconductor medium. Assume that a laser diode is coupled with the external cavity whose optical length nRLR decreases with an increase in the temperature, the condition that makes the total optical length (nDLD+nRLR) of the cavity constant regardless of the temperature is given by the following expression (2).
                              ∂                      /                          ∂                              T                ⁡                                  (                                                                                    n                        D                                            ⁢                                              L                        D                                                              +                                                                  n                        R                                            ⁢                                              L                        R                                                                              )                                                                    =                                                            L                D                            ⁢                                                ∂                                      n                    D                                                  /                                  ∂                  T                                                      +                                          n                D                            ⁢                                                ∂                                      L                    D                                                  /                                  ∂                  T                                                      +                                          L                R                            ⁢                                                ∂                                      n                    R                                                  /                                  ∂                  T                                                      +                                          n                R                            ⁢                                                ∂                                      L                    R                                                  /                                  ∂                  T                                                              =          0                                    (        2        )            Note ∂nR/∂T and ∂LR/∂T become negative because ∂nD/∂T and ∂LD/∂T are usually positive.
Here, to splice the waveguides with different refractive indices, such as splicing the semiconductor optical waveguide with the waveguide composed of non-semiconductor materials, reflection occurs at the interface because of the difference between the refractive indices of the two waveguides. Assume that the refractive index of a first optical waveguide is N1, and the refractive index of a second optical waveguide is N2, and consider the plane wave for simplicity, then the reflectance R is given by the following expression (3).R=((N1−N2)/(N1+N2))2  (3)
When light propagated through the semiconductor or silica waveguide is radiated to the outside, the reflection occurs because of the difference between the refractive index of the waveguide and that of the outside. Accordingly, when the light propagated through the semiconductor optical waveguide is radiated to the air from an end face of the semiconductor laser, for example, the reflection can be prevented by forming an evaporated film with a certain thickness on the semiconductor end face as disclosed in Toru Kusakawa, “Lens optics”, pp. 273-288, the Tokai University Press. However, it is difficult to form such an antireflection film at high accuracy when integrating the waveguide composed of different materials on a semiconductor substrate.
On the other hand, when incident light is entered at an angle into the interface surface between materials with different refractive indices, refraction occurs at the interface surface as represented by the following expression (4) according to Snell's law,sin θ1/sin θ2=N2/N1  (4)where θ1 is an incident angle and θ2 is a refraction angle.
When the incident angle θ1 equals the Brewster angle θB, the reflection of the components parallel to the incidence plane can be eliminated, in which case, the Brewster angle θB is represented by the following expression (5).θB=tan−1(N2/N1)  (5)
Now, the semiconductor waveguide usually employ a buried heterostructure or ridge structure. As for the etching and buried growth of the semiconductor, there is a crystal orientation suitable for the etching and buried growth.
When splicing the semiconductor optical waveguide with the optical waveguide composed of the materials whose refractive index differs from the materials of the semiconductor optical waveguide, the reflection occurs at the splice interface in accordance with the difference between the refractive indices, thereby limiting the flexibility of the waveguide design.
Although the reflection can be reduced at the interface between the waveguides having different refractive indices by utilizing the Brewster angle θB, the use of the Brewster angle θB causes the light to refract through the interface surface between the waveguides, which presents a problem in that the waveguides are no longer aligned.
Also, when utilizing the Brewster angle θB to reduce the reflection between the waveguides with different refractive indices, it becomes difficult to configure the buried semiconductor waveguide along with a certain crystal orientation, which makes it difficult to fabricate the buried semiconductor waveguide at a high reliability.
Furthermore, when utilizing the Brewster angle θB to reduce the reflection between the waveguides with different refractive indices, it becomes difficult to place the semiconductor waveguides perpendicularly to a cleaved surface, which precludes the cleaved surface to be used as the reflection plane of the semiconductor laser.
As described above, combining the materials that differ in the refractive indices and their temperature dependence poses a variety of problems and is desired to be improved further.